Intracellular delivery of HIV and SIV expression vectors within the epidermis of rhesus macaques using the Accell TM gene delivery system resulted in the induction of significant IgG responses using as little as one microgram of DNA per immunization. These responses were greatly enhanced following the administration of either recombinant subunit or recombinant vaccinia virus booster immunizations. Five monkeys primed with constructs expressing either HIV-1(LAI)gag-pol-env or gp120 were boosted with two doses of rgp120(SF2) and rp24 adjuvanted with MD59. All five animals showed dramatically elevated antibody responses after a similar subunit immunization, and only developed modest titers after a second subunit immunization (1:170,000). In contrast, animals that were primed with the relevant DNA did not develop immunization (1:6,000). Another set of three monkeys were primed with several consecutive doses of DNA encoding SIVmac239gp160 and developed maximal IgG titers of approximately 1:20,000. Upon receipt of a single booster immunization containing a live recombinant vaccinia virus expressing SIVmac239gp160, these animals developed endpoint IgG titers specific for SIV gp160 of > 1:2,000,000. These responses were dramatically higher than those seen in parallel animals immunized with either DNA or vaccinia vectors alone. In addition, animals that were first primed with the vaccinia vector, also developed vigorous antibody responses after the receipt of a gene vaccine administration. All vaccinated monkeys and 5 naive controls were challenged intravenously with a 10 minimum-infectious doses of SHIVSF33. All monkeys became infected, with vaccinates displaying virus loads and rates of disease progression comparable to the naive controls.